1. Field of the Invention
The present invention relates to a method for manufacturing a nonvolatile semiconductor memory device storing an electron in a floating gate, and is particularly used in a memory device having a large capacity such as an EPROM, an EEPROM, and an FEEPROM.
2. Description of the Related Art
Conventionally, a nonvolatile semiconductor memory device, which stores an electron in a floating gate, such as a memory cell of an EPROM has a cross sectional structure as shown in FIG. 1. In FIG. 1, reference numeral 201 is a silicon substrate; 202: a source or drain diffusion layer; 203: a gate oxide film; 204: a floating gate; 205: a control gate; 206: a thermal oxide film; and 207: an interlayer insulating film.
The thermal oxide film 206 is formed around the floating gate of the EPROM to maintain the electric charge in the floating gate. The thermal oxide film 206 is formed by thermally oxidizing the floating gate, which is formed of polycrystalline silicon. Additionally, the thermal oxide film 206 having a sufficient thickness is formed at a high temperature in order to keep a characteristic of maintaining favorable electrical charge.
The formation of the thermal oxidation film 206 having a sufficient thickness at a high temperature is to prevent the following problems.
It cannot be said that the quality of the thermal oxide film 206, which is formed on the side surface of the floating gate 204, is high. Due to this, if the distance between the edge portion of the floating gate 204 and the source or drain diffusion layer 202 is close to each other, high electrical filed is applied between the edge portion and the the source or drain diffusion layer 202, so that the electric charge in the floating gate 204 is easily moved out of the floating gate. Particularly, if the edge portion of the floating gate 204 is sharpened to form an acute angle, the electrical field is further concentrated on the portion between the edge portion and the the source or drain diffusion layer 202, so that electric charge is easily moved out of the floating gate. Due to this, the thermal oxide film 206 having a sufficient thickness is formed at a high temperature, and the edge portion of the floating gate 204 is rounded, thereby controlling the electric charge passing through a passage 1. Moreover, a large amount of phosphorus (P) is contained in the interlayer insulating film 207 (e.g., PSG film) formed on the thermal oxide film 206. In other words, the quality of the interlayer insulating film 207 is low and the electric charge is easily passed through. Due to this, the thermal oxide film 206 having a sufficient thickness is formed, thereby controlling the electric charge passing through a passage 2. Moreover, there is a case in which the floating gate 204 contains a large amount of impurity materials and its side surface is often contaminated. Due to this, in order to improve the quality of the thermal oxide film 206, it is required that the thermal oxide film 206 be formed to have a sufficient thickness at a high temperature.
However, if the thermal oxide film 206 is oxidized at a high temperature for a long time, the contact surface between the substrate 201 and the source or drain diffusion layer 202 is formed at a deep position. Due to this, there is a problem in that the characteristic of the device is deteriorated. Moreover, if the oxidation time is too long, the thermal oxide film 206 enters between the floating gate and the control gate 205, so that the thickness of a thermal oxidation film 208 between the floating gate 204 and the control gate 205 is increased. In this case, there is a problem in that the capacity between the floating gate 204 and the control gate 205 decreases and the characteristic of the device decreases. Particularly, the decrease in the capacity largely influences on a device such as a fined memory cell having a short channel length.